Cavitation barrier for aquatic species

ABSTRACT

Embodiments of the present invention provide a novel deterrent barrier based on the phenomenon of fluid cavitation. A drive unit comprising a motor and a propeller are configured for inducing cavitation in water. The cavitation takes the form of a rotationally confined vertical column of cavitation bubbles extending from the propeller, and a one-dimensional series of drive units spanning the width of a waterway may provide an effective, environmentally friendly and non-lethal barrier against entry of target fish species.

RELATED APPLICATION

This application claims priority to, and the benefits of, U.S.Provisional Application Ser. No. 62/754,264, filed on Nov. 1, 2018, theentire disclosure of which is hereby incorporated by reference.

BACKGROUND

In the United States, asian carp refers to grass, silver, bighead, andblack carp species. These fish originated in eastern Asia. Grass carpwere introduced into United States waters to help control weeds inaquaculture operations. Silver and bighead carp, which feed on plankton,and black carp, which are molluscivores, were undesirable tag-alongs.These voracious fish soon spread and have been crowding out native fishpopulations. Increasing carp populations are also altering ecosystemsand killing off sensitive species such as freshwater mussels. Fishermenusing young carp as live bait have further increased the spread of Asiancarp, as have boats traversing locks up and down the Mississippi River.

Asian carp are hardy, lay hundreds of thousands of eggs at a time, andspread into new habitats quickly and easily. They can jump over barrierssuch as low dams. Conventional exclusion structures, such as underwaterfences (including an elaborate electric fence) not only have failed toprevent further spread of the carp population, but are unwieldy,expensive, and potentially harmful to other species, navigation, and thenatural aquatic environment.

SUMMARY

Embodiments of the present invention provide a novel deterrent barrierbased on the phenomenon of fluid cavitation. The cavitation barrieroffers a sustainable, long-term approach to exclusion of carp and otheraquatic species (including aquatic mammals) from waterways. Thecavitation barrier poses no threat to humans, wildlife, navigation, orinfrastructure. The cavitation zone is tightly confined, and humans andanimals can readily detect and avoid it. Ships may pass through thebarrier without impairing its ability to deter fish. Cavitation bubblesdo not change water chemistry and will not increase corrosion of metalstructures like gates and dams.

Accordingly, in a first aspect, the invention relates to a deterrentbarrier system for aquatic species. In various embodiments, the systemcomprises a mounting plate configured for affixation to a submergedfeature of a waterway; on the mounting plate, at least drive unitcomprising a motor and a propeller configured for inducing cavitation inwater, the cavitation taking the form of a rotationally confinedvertical column of cavitation bubbles extending from the propeller; anda protective grid substantially enclosing the propeller. The cavitationbubbles may take the form of vertical streams.

In some embodiments, the system comprises a plurality of drive unitsarranged transversely across the width of the waterway to generate aseries of vertical columns of cavitation bubbles collectively forming awaterway-spanning curtain. The system may include a controller foroperating the drive units so as to create regions in the curtain havingweaker cavitation strength to permit traversal of the curtain bynon-target aquatic species. The controller may be configured to operatethe drive units in a pulsed fashion. In various embodiments, each of thedrive units includes a plurality of motors (e.g., two) configured forrotating a single propeller.

In another aspect, the invention pertains to a method of selectivelydeterring migration of a target aquatic species along a waterway. Invarious embodiments, the method comprises the steps of providing acavitation-producing drive unit; and operating the drive unit to inducecavitation in water, the cavitation taking the form of a rotationallyconfined vertical column of cavitation bubbles extending from the driveunit, the cavitation bubbles repelling the target aquatic species.

A plurality of drive units may be arranged transversely across the widthof the waterway to generate a series of vertical columns of cavitationbubbles collectively forming a waterway-spanning curtain. The cavitationbubbles take the form of vertical streams, and the drive units may bedriven so as to create regions in the curtain having weaker cavitationstrength to permit traversal of the curtain by non-target aquaticspecies.

In some embodiments, the drive units are operated in a pulsed fashion.The method may, in some cases, further comprise the steps ofcomputationally detecting the target aquatic species upstream from thedrive unit; and in response, sending an activation signal to the driveunit, the drive unit remaining inactive until receipt of the activationsignal.

The method may, in some instances, further include computationallydetecting other aquatic species upstream from the drive unit, and inresponse, sending an activation signal to the drive unit to operate in apulsed mode. In some embodiments, the method further includes the stepsof providing a plurality of additional cavitation-producing units, thecavitation-producing units being arranged transverse to the waterway;computationally detecting other aquatic species upstream from the driveunits; and in response, sending an activation signal to the drive unitsto create a semi-continuous barrier allowing passage of non-targetaquatic species.

The drive unit may include a motor and a propeller, a high-pressure jet,a piezoelectric transducer, an optical source, and/or a source offocused ultrasound.

The term “substantially” or “approximately” means ±10% (e.g., by weightor by volume), and in some embodiments, ±5%. Reference throughout thisspecification to “one example,” “an example,” “one embodiment,” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present technology. Thus, the occurrences ofthe phrases “in one example,” “in an example,” “one embodiment,” or “anembodiment” in various places throughout this specification are notnecessarily all referring to the same example. Furthermore, theparticular features, structures, routines, steps, or characteristics maybe combined in any suitable manner in one or more examples of thetechnology. The term “consists essentially of” means excluding othermaterials that contribute to function, unless otherwise defined herein.Nonetheless, such other materials may be present, collectively orindividually, in trace amounts. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be more readily understood from the followingdetailed description of the invention, in particular, when taken inconjunction with the drawings, in which:

FIG. 1 schematically illustrates the phenomenon of cavitation.

FIG. 2 is a schematic plan view illustrating the basic operation of anembodiment of the invention.

FIG. 3 is a perspective view of a representative propeller useful inconnection with various embodiments and the propeller's operation.

FIG. 4 is a schematic plan view illustrating a partial barrier generatedin accordance with embodiments of the invention.

FIG. 5 is a schematic plan view illustrating use of a partial barrier inconjunction with pulsed operation and one or more optional nets.

FIG. 6 is a sectional schematic elevation illustrating the components ofa representative installation in accordance with embodiments of theinvention.

FIG. 7 is a sectional schematic elevation illustrating a suitablemounting system for the installation shown in FIG. 6 .

DETAILED DESCRIPTION

Cavitation is the formation of voids in a liquid when pressure rapidlychanges within it. The fluid spontaneously “boils” in regions of reducedpressure, creating bubbles that implode when they travel to regions ofhigher pressure. Bubble collapse can be highly energetic, generatingtemperatures as high as 19,700° C., acoustic shockwaves, and liquid jetsthat can erode metal. FIG. 1 illustrates successive stages in thecollapse of a cavitation bubble. Cavitation may occur due to mechanicalacceleration of the liquid or its passage through a restriction. Whenfluid is accelerated, the local pressure decreases in accordance withBernoulli's Principle, and a vapor bubble 100 may form when the localpressure falls below the liquid's vapor pressure. When the velocity ofthe liquid decreases downstream of the disturbance or restriction, thepressure recovers, causing the bubble 100 to implode in a sequence ofstages 105 that collectively represent the cavitation phenomenon. Thecollapsing bubble draws a jet of liquid at supersonic speed, and in thefinal stage that jet is ejected from the bubble at high speed,potentially causing ablation of nearby surfaces.

With reference to FIG. 2 , an embodiment of the invention includes aplurality of high-speed propellers 200 to create rotationally confinedcolumns 205 of cavitation bubbles spanning a waterway. These columns maybe vertical, horizontal, or inclined at an angle. Each propeller 200sweeps out a region of, for example, 1 m in diameter and generatesthrust to project bubbles upward. A row of propellers 200 forms a wallof bubbles, desirably across the entire transverse width of thewaterway, and the propellers may be screened by a barrier 210—forexample, a metal, polymer or composite screen, cage or grid—to preventinjury to fish. The density of the bubble curtain at a 1 m cavitationzone width ensures that fish as small as two inches will interact withthe bubble field. Accordingly, the approach of the invention is tocreate a non-physical barrier that avoids expensive, intrusivestructures that can interfere with navigation and the local ecologywhile effectively excluding species such as asian carp.

In particular, deterrence of carp is achieved in multiple ways.Implosion of the bubbles on the skin of the fish causes non-lethaldiscomfort. The complex acoustic signals that accompany cavitationprovide an additional negative stimulus. Within each cavitation zone205, the presence of bubbles reduces the buoyancy and swimming abilityof fish, restricting their passage. Moreover, the bubble-filled regioncollectively formed by the columns 205 concentrates sound, because ithas a different density than the surrounding water. The barrier 205introduces no toxic chemicals into the water and does not alter thewater chemistry. The bubbles are true voids, containing water vapor atextremely low pressure. Because they are created from the water itself,they are chemically inert (unlike CO₂ bubbles, for example) and confinedto a discrete region.

The cavitation bubbles form continuous streams accelerated upwards bythe propellers 200 and their own buoyancy. To the unaided eye (human orfish), the helical streams are not visible; the bubbles have theappearance of a continuous cylindrical sheet. The spiral motion impartedby the propellers 200 ensures that the bubble columns 205 retain theirvertical alignment substantially all the way to the surface. The columns205 may appear as a wall of silver pillars. It is also important to notethat the effects of bubble collapse (free jets and shockwaves) are feltat a finite distance from the columns 205. Water is incompressible, andshock waves from bubble implosion will cause discomfort even if a fishdoes not directly touch the bubbles. The long range of the sound and thepropagation of the shock fronts will repel small and large fish.

The propeller blades are designed to produce stable columns of bubbleswhile minimizing wear and energy consumption. The blades are made fromaustenitic steel alloys with a high proportion of chromium (15-20%) fordurability and corrosion resistance. Long blades have a higher linearvelocity for a given angular velocity. In accordance with Bernoulli'sPrinciple, high fluid speed reduces pressure, and cavitation requiresattainment of a sufficiently low pressure. In general, sharper bladescreate low pressure more easily, enabling bubbles to form at lowerrotational speeds and reducing energy consumption (although as explainedherein, there may be an optimal bubble size to deter aquatic species ofinterest). FIG. 3 illustrates a representative cavitation propeller 300with blades pitched to promote clean detachment of bubbles; thisconfiguration extends blade life, as properly designed blades will sweepthe minimum volume necessary to carry bubbles to the water surface. Asnoted above, the helical column of bubbles 310 created by propellerrotation 300 retains its shape as the column rises.

As shown in FIG. 4 , a partial barrier 400 (i.e., one not fully spanningthe width of the waterway) may be formed to permit other fish to migratewhile still making the region inhospitable to target species such ascarp. Alternatively, differential sensitivities of fish species to noisecan be exploited to create semi-continuous barriers that preferentiallyexclude some fish but not others. In the regions 420, the barrierstrength is relatively strong (i.e., the bubble density is relativelyhigh), whereas in the regions 430, the barrier strength is weaker (i.e.,the bubble density is relatively low). Barrier strength can becharacterized using optical transmission, sound intensity, and pressure.These properties will vary with depth and water temperature.

For example, as light as light travels from the denser medium (water) tothe less-dense medium (water vapor in the cavitation bubble), totalinternal reflection occurs at a critical angle of 48.6°. This propertycan be used to measure barrier strength. In particular, a high barrierstrength may be defined by transmission of less than 50% of lightincident on the barrier at the water/water-vapor critical angle, and alow barrier strength may be defined by transmission of at least 50% oflight incident on the barrier at the water/water-vapor critical angle.

In the far field, whose outer range is representatively indicated at440, the acoustic fields of the regions 420, 430 add constructively topresent an effectively continuous barrier. This repels carp. In the nearfield of the barrier, whose boundary is representatively indicated at450, fish less deterred by the noise detect the gap indicated by thearrow. The sound and bubble production can be precisely controlled atthe location of each propeller 200, 300 by changing the power supplied.

Another strategy to preferentially target carp, illustrated in FIG. 5 ,is pulsed operation of the cavitation barrier. Carp are hearingspecialist fish and have auditory thresholds of 60 dB at 1 kHz.Propeller cavitation can generate sound levels exceeding 200 dB. Loudernoise drives carp into the nets 510, while less sensitive fish are leftbehind and do not encounter the nets. In particular, desirable fishforage closer to the barrier and remain at a greater distance from thenets 510. Their weaker response to the sound means that “by-catch” willbe reduced, and carp removal will be efficient and selective.

A representative system 600, utilizing a single propeller forillustrative purposes, is shown in FIG. 6 . A propeller 610, driven byan electric motor 617, creates a helical column of bubbles. A secondelectric motor 622 may be included for redundancy, but each motorprovides sufficient torque to produce a stable bubble column. Theoperation and performance of the motor(s) is monitored by a controller625, which may be connected to the motor(s) by a wired or, in somecases, wireless connection. In some embodiments, the controller 625 isconnected (e.g., via the internet or telecommunications infrastructure)to a command server that monitors the operation of one or manyfish-barrier installations in accordance herewith. The command servermay itself respond to periodic updates from sensors, cameras, and/orhuman agents who report the presence of undesirable fish in variouswaterways. Based on this stream of real-time information, the commandserver issues control signals to various controllers 625 so that themotors operate only where needed and in the appropriate mode. Forexample, autonomous cameras may operate in conjunction withcomputer-vision systems and machine-learning classifiers (e.g.,convolutional neural networks) that recognize different fish species andsend continuous updates to the command server. If the command serverreceives a report of Asian carp traveling along a monitored waterway, itmay signal controllers 625 associated with that waterway and/ortributaries thereto to impede migration of carp to aquatic environmentsthat would be adversely affected. If, however, the command serverreceives reports of mixed fish populations, it may signal thecontrollers to operate their associated drive units in pulsed mode or tocreate semi-continuous barriers so as to preferentially exclude carp orother harmful species. When carp migration ceases, the command servermay signal the controllers to shut off their associated drive units. Inthis way, systems are operated individually, only when needed, and insituationally appropriate modes.

The controller 625 may be provided as either software, hardware, or somecombination thereof. For example, the system may be implemented on oneor more conventional computers including one or more processors such asthe Pentium or Celeron family of processors manufactured by IntelCorporation of Santa Clara, Calif. The processor may also include a mainmemory unit for storing programs and/or data. The memory may includerandom access memory (RAM), read only memory (ROM), and/or Flash memoryresiding on commonly available hardware such as one or moreapplication-specific integrated circuits (ASIC), field-programmable gatearrays (FPGA), electrically erasable programmable read-only memories(EEPROM), programmable read-only memories (PROM), or programmable logicdevices (PLD). For embodiments in which the control functions areexecuted by one or more software programs, the programs may be writtenin any of a number of high level languages such as PYTHON, FORTRAN,PASCAL, JAVA, C, C++, C #, BASIC, various scripting languages, and/orHTML. The software may be embodied on an article of manufactureincluding, but not limited to, a hard disk, an optical disk, a magnetictape, a PROM, an EPROM, EEPROM, field-programmable gate array, orCD-ROM.

The controller 625 may include various additional conventional elementssuch as one or more mass storage devices, one or more input/output (I/O)ports to receive signals from sensors deployed to monitor motorfunction, a communication platform including a network interface tofacilitate wireless and/or wired communications over a computer networkor the telecommunications infrastructure, an input device, and adisplay. As an alternative or addition to the input device and display,an interface module may be included to permit a user to issue commandsand view data via the wireless communication platform using, forexample, a smart phone or tablet. In some embodiments, the controller625 implements a webserver, facilitating remote access and control overthe internet via IP and TCP/IP protocols (see, e.g., U.S. Pat. No.6,201,996, the entire disclosure of which is hereby incorporated byreference). Local communication may take place via WiFi, Bluetooth,ZigBee, IrDa or other suitable protocol. Furthermore, components of thesystem may communicate through a combination of wired or wireless paths.

With continued reference to FIG. 6 , a protective (typically metal) grid630 surrounds the motor(s) and the propeller 610, and is anchored, alongwith the motor(s), to a base plate 635 that is itself affixed to asubmerged concrete base 640. The motor(s) may be affixed as a unit to amounting plate 645 anchored to the base plate 635 by a series of anchors(e.g., four or more) 650. One suitable mounting arrangement 700 is shownin FIG. 7 . The mounting plate 645 and the base plate 635 havecomplementary alignment features 710 a, 710 b. The plates 635, 645, oncealigned by mating of the features 710 a, 710, are permanently affixed tothe concrete base 640 by the anchors 650, which are driven throughaligned apertures 720, 725 in both plates 635, 645 and therebelow intothe concrete base. A circular magnet 730 surrounding each aperture 720of the mounting plate 645 and an additional circular magnet 735thereunder surrounding the aperture 725 of the base plate 635 allow forconvenient placement of a washer 735 and the anchor 650 and enforcevertical alignment of the anchor 650 as it is driven into the concretebase 640.

Deploying propellers as individual modular units permits individualmodules to be replaced without interrupting system operation, since mostof the barrier will remain active. Depending on the overall number ofunits, the barrier will still operate as an acoustic deterrent even if afew units are disabled.

Various alternatives or enhancements are possible. For example,cavitation regions may be created using high-pressure jets,piezoelectric transducers, optical sources, or focusedultrasound—collectively indicated at 600 in FIG. 6 —instead of or inaddition to propellers. Suitable equipment for implementing thesealternative approaches to cavitation is conventional or readily obtainedor designed by those skilled in the art without undue experimentation.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A method of selectively deterring migration of atarget aquatic species along a waterway having a width, the methodcomprising the steps of: providing, arranged transversely across thewidth of the waterway, a plurality of cavitation-producing drive unitseach including a propeller; and operating the drive units to (i)directly induce cavitation in water, the propellers being configured toproduce a series of spiral, rotationally confined vertical columns ofcavitation vapor bubbles extending from the drive units and collectivelyforming a curtain spanning the width of the waterway, the curtainrepelling the target aquatic species and preventing traversal throughthe curtain by the target aquatic species, and (ii) create regions inthe curtain having weaker cavitation strength to permit traversalthrough the curtain by a non-target aquatic species while stillpreventing traversal through the curtain by the target aquatic species.2. The method of claim 1, wherein the cavitation bubbles manifest asvertical streams.
 3. The method of claim 1, further comprising the stepof operating the drive units in a pulsed fashion.
 4. The method of claim1, further comprising the steps of: computationally detecting the targetaquatic species upstream from the drive units; and in response, sendingan activation signal to the drive units, the drive units remaininginactive until receipt of the activation signal.
 5. The method of claim1, further comprising, prior to operating the drive units to createregions in the curtain having weaker cavitation strength to permittraversal through the curtain by the non-target aquatic species:computationally detecting a mixed population of aquatic species upstreamfrom the drive units, the mixed population comprising the target aquaticspecies and the non-target aquatic species.
 6. The method of claim 1,wherein each drive unit comprises a motor and a propeller.
 7. The methodof claim 1, wherein each drive unit comprises a high-pressure jet. 8.The method of claim 1, wherein each drive unit comprises a piezoelectrictransducer.
 9. The method of claim 1, wherein each drive unit comprisesan optical source.